NSW Maritime spokesperson Alex Barrell said conditions are expected to deteriorate from tomorrow Monday July 13 due to an east coast low pressure system forming off the coast.

"Between 150 - 300 millimeters of rainfall may fall in some areas between Monday and Wednesday accompanied by strong gale force winds," Mr Barrell said.

"The seas are likely to be very rough and localised flash flooding is expected which may impact the South Coast and will be more severe if the trough moves north to the Central Coast.

"Although it is winter and there are fewer boaters on our waterways, it is school holidays and skippers need to remain vigilant if children are on board.

"Everyone should wear a lifejacket at all times and, if bad weather impacts your trip, skippers should find a well-protected waterway as a Plan B, or not head out on the water at all.

"Skippers and owners of small boats should be on high alert and check the conditions before considering crossing a coastal bar or heading offshore.

"While conditions are expected to ease by Wednesday, large ocean swells are likely to persist for some days and crossing ocean bars will remain a challenge for navigation."

"NSW Maritime strongly recommends reconsidering your need to be out on the water in those affected areas over the next few days."

Every skipper is responsible for the safety of their vessel and everyone on board.

NSW Maritime and Centre for Maritime Safety offer the following safety tips include: